Power converter

ABSTRACT

A power converter includes: a power converter main circuit that includes semiconductor switching elements; gate drive circuits driving the semiconductor switching elements, respectively; and one or a plurality of impedance element groups connected between at least one pair of the gate drive circuits. At least one of the gate drive circuits includes a detector that detects a voltage across the impedance element group, and changes the driving speed of the semiconductor switching elements in accordance with an output of the detector.

FIELD

The present invention relates to a power converter that incorporatestherein a power semiconductor switching element.

BACKGROUND

A power converter such as an inverter, a servo amplifier, or a switchingpower supply incorporates therein one or a plurality of powersemiconductor switching elements. The power semiconductor switchingelement undergoes a change in the conduction state between a first mainterminal and a second main terminal in accordance with an electricalsignal applied between a first signal input terminal and a second signalinput terminal. A gate drive circuit receives a command signal from anupper controller to apply an electrical signal between the first signalinput terminal and the second signal input terminal of the powersemiconductor switching element and drive the power semiconductorswitching element.

When the power semiconductor switching element is off, a current doesnot flow between the first main terminal and the second main terminaleven with a high voltage applied therebetween. However, an excessivelyhigh voltage applied to the power semiconductor switching element causesa failure of the power semiconductor switching element. A technique hasthus been proposed in which a voltage of each part of the powerconverter is detected and transmitted to the gate drive circuit. Thegate drive circuit performs an operation that changes a method ofdriving the power semiconductor switching element on the basis ofinformation on the transmitted voltage.

With the background art described above, Patent Literature 1 belowpresents a motor drive system for an electric vehicle. A voltagedetection circuit is attached to a DC power supply in the motor drivesystem to detect a voltage. The voltage detection circuit transmits asignal to the gate drive circuit when the voltage of the DC power supplyis higher than or equal to a predetermined voltage. Upon receiving thesignal, the gate drive circuit changes the connection configuration of agate resistor and changes the method of driving the power semiconductorswitching element. The gate drive circuit prevents excessive generationof a surge voltage when the power semiconductor switching element isturned on and turned off. This prevents the application of anexcessively high voltage to the power semiconductor switching element.

In Patent Literature 2 listed below, a first main terminal of a powersemiconductor switching element and a gate drive circuit are connectedby wiring. This wiring allows transmission of the voltage between thefirst main terminal and a second main terminal of the powersemiconductor switching element to the gate drive circuit. The gatedrive circuit detects the voltage between the first main terminal andthe second main terminal of the power semiconductor switching element.The gate drive circuit changes the resistance value of a built-inresistor in accordance with the voltage between the first main terminaland the second main terminal to change the method of driving the powersemiconductor switching element and prevent excessive generation of thesurge voltage when the power semiconductor switching element is turnedon and turned off. This prevents the application of an excessively highvoltage to the power semiconductor switching element.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No. H9-23664

Patent Literature 2: Japanese Patent Application Laid-Open No. H6-291631

SUMMARY Technical Problem

The configurations as disclosed in Patent Literatures 1 and 2 allow thevoltage of each part of the power converter to be detected andtransmitted to the gate drive circuit. However, a detection target thatis the voltage of each part of the power converter is detected at aposition away from the position of the gate drive circuit. This causes aproblem in that noise is superimposed before voltage information on eachpart of the power converter reaches the gate drive circuit.

The present invention has been made in view of the aforementionedproblem, and an object of the present invention is to obtain a powerconverter capable of reducing the influence of noise when voltageinformation on each part of the power converter is transmitted to a gatedrive circuit.

Solution to Problem

In order to solve the aforementioned problem and achieve the object, apower converter according to an aspect of the present invention includesa power converter main circuit including two or more semiconductorswitching elements; gate drive circuits, each of which drives acorresponding one of the semiconductor switching elements; and one or aplurality of impedance elements connected between at least one pair ofthe gate drive circuits. At least one of the gate drive circuitsincludes a detector to detect a voltage across the impedance elements ora current flowing through the impedance elements, and changes a drivingspeed of the semiconductor switching elements in accordance with anoutput of the detector.

Advantageous Effects of Invention

The present invention exhibits an effect in that it is possible toreduce the influence of noise when the voltage information on each partof the power converter is transmitted to the gate drive circuit.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram illustrating the configuration of a mainpart of a power converter according to a first embodiment.

FIG. 2 is a state transition diagram illustrating a change in an outputvoltage when a power converter main circuit according to the firstembodiment performs a sink operation.

FIG. 3 is a state transition diagram illustrating a change in an outputvoltage when the power converter main circuit according to the firstembodiment performs a source operation.

FIG. 4 is a circuit diagram illustrating the configuration in which adetector is provided in only one gate drive circuit as a modification ofthe power converter according to the first embodiment.

FIG. 5 is a circuit diagram illustrating the configuration of a mainpart of a power converter according to a second embodiment.

FIG. 6 is a circuit diagram illustrating the configuration of a mainpart of a power converter according to a third embodiment.

FIG. 7 is a circuit diagram illustrating one mode of the operation ofthe power converter according to the third embodiment.

FIG. 8 is a circuit diagram illustrating a mode of the operation of thepower converter according to the third embodiment, the mode beingdifferent from that of FIG. 7.

FIG. 9 is a circuit diagram illustrating a mode of the operation of thepower converter according to the third embodiment, the mode beingdifferent from that of each of FIGS. 7 and 8.

FIG. 10 is a circuit diagram illustrating the configuration of a mainpart of the power converter according to the third embodiment, theconfiguration being different from that of FIG. 6.

FIG. 11 is a circuit diagram illustrating the configuration of a mainpart of the power converter according to the third embodiment, theconfiguration being different from that of each of FIGS. 6 and 10.

FIG. 12 is a circuit diagram illustrating the configuration of a mainpart of the power converter according to the third embodiment, theconfiguration being a combination of those of FIGS. 6, 10, and 11.

FIG. 13 is a circuit diagram illustrating the configuration of a mainpart of a power converter according to a fourth embodiment.

FIG. 14 is a circuit diagram illustrating one mode of the operation ofthe power converter according to the fourth embodiment.

FIG. 15 is a circuit diagram illustrating a mode of the operation of thepower converter according to the fourth embodiment, the mode beingdifferent from that of FIG. 14.

FIG. 16 is a circuit diagram illustrating a mode of the operation of thepower converter according to the fourth embodiment, the mode beingdifferent from that of each of FIGS. 14 and 15.

FIG. 17 is a circuit diagram illustrating a mode of the operation of thepower converter according to the fourth embodiment, the mode beingdifferent from that of each of FIGS. 14 to 16.

FIG. 18 is a circuit diagram illustrating the configuration of a mainpart of the power converter according to the fourth embodiment, theconfiguration being different from that of FIG. 13.

FIG. 19 is a circuit diagram illustrating the configuration of a mainpart of the power converter according to the fourth embodiment, theconfiguration being different from that of each of FIGS. 13 and 18.

DESCRIPTION OF EMBODIMENTS

A power converter according to embodiments of the present invention willnow be described with reference to the accompanying drawings. Note thatthe present invention is not limited by the following embodiments.

First Embodiment

FIG. 1 is a circuit diagram illustrating the configuration of a mainpart of a power converter according to a first embodiment. As theconfiguration of the main part of the power converter according to thefirst embodiment, FIG. 1 illustrates a load 2 to be driven; a powerconverter main circuit 10 that drives the load 2; gate drive circuits12A and 12B that are peripheral circuits that control the powerconverter main circuit 10; insulating circuits 14A and 14B; an impedanceelement group 16; a switching signal generation unit 20; and a capacitor6 that is a power supply source for the power converter main circuit 10and accumulates DC power.

The power converter main circuit 10 of the first embodiment isconfigured to include a semiconductor switching element 10A as a firstpower semiconductor switching element connected to a DC bus 7A on a highpotential side; and a semiconductor switching element 10B as a secondpower semiconductor switching element connected to a DC bus 7B on a lowpotential side. The semiconductor switching element 10A and thesemiconductor switching element 10B are connected in series, and theload 2 is connected to the electrical connection point of the elements.

The semiconductor switching element 10A is provided with a first mainterminal 10A1, a second main terminal 10A2, a first signal inputterminal 10A3, and a second signal input terminal 10A4. Likewise, thesemiconductor switching element 10B is provided with a first mainterminal 10B1, a second main terminal 10B2, a first signal inputterminal 10B3, and a second signal input terminal 10B4. In the powerconverter main circuit 10 configured as described above, the first mainterminal 10A1 of the semiconductor switching element 10A is connected tothe DC bus 7A, the second main terminal 10A2 of the semiconductorswitching element 10A is connected to the first main terminal 10B1 ofthe semiconductor switching element 10B, and the second main terminal10B2 of the semiconductor switching element 10B is connected to the DCbus 7B.

The DC bus 7A is connected to an upper DC terminal 8A of the capacitor6, and the DC bus 7B is connected to a lower DC terminal 8B of thecapacitor 6. That is, the voltage of the capacitor 6 is applied betweenthe DC buses 7A and 7B.

In the power converter main circuit 10 connected as described above, thepotential of the DC bus 7A is applied to the load 2 when thesemiconductor switching element 10A becomes conductive, and thepotential of the DC bus 7B is applied to the load 2 when thesemiconductor switching element 10B becomes conductive. The powerconverter main circuit 10 thus outputs two kinds of potentials, i.e.,the potential of the DC bus 7A or the potential of the DC bus 7B,thereby operating as a two-level power converter circuit.

In each of the semiconductor switching elements 10A and 10B, atransistor element and a diode element are connected in parallel. Notethat connection of the diode element in each of the switching elementsmay be omitted depending on the characteristic of the load such as whenthe load is a resistive load.

Although FIG. 1 illustrates a MOSFET as the transistor element, thetransistor element is not limited to the MOSFET but may be any devicethat can switch a state between a low resistance state and a highresistance state by using an electrical signal. For example, an IGBT ora bipolar transistor may be used as the transistor element. Moreover, awide band-gap semiconductor such as SiC, GaN, or diamond in addition toSi which is used widely may be used as a material of the transistorelement and the diode element that make up each of the semiconductorswitching elements 10A and 10B.

The gate drive circuit 12A is a first gate drive circuit that drives thesemiconductor switching element 10A that is the first powersemiconductor switching element. The gate drive circuit 12B is a secondgate drive circuit that drives the semiconductor switching element 10Bthat is the second power semiconductor switching element. The gate drivecircuit 12A has the same configuration as the gate drive circuit 12B;therefore, the internal configuration of the circuits will be describedwith reference to the gate drive circuit 12A.

The gate drive circuit 12A includes four bridged transistor elements,specifically a first on transistor 12A1 a, a first off transistor 12A1b, a second on transistor 12A1 c, and a second off transistor 12A1 d.The first on transistor 12A1 a and the first off transistor 12A1 b areconnected in series via two gate resistors 12A2 a and 12A2 b, and thesecond on transistor 12A1 c and the second off transistor 12A1 d areconnected in series via two gate resistors 12A2 c and 12A2 d. Theconnection point of the gate resistors 12A2 a and 12A2 b and theconnection point of the gate resistors 12A2 c and 12A2 d are connectedto each other to be connected to the first signal input terminal 10A3 ofthe semiconductor switching element 10A.

The gate drive circuit 12A further includes a switching speed changingunit 12A3 that changes the speed at the time of driving thesemiconductor switching element 10A that is the first powersemiconductor switching element. The switching speed changing unit 12A3can be configured by a logic circuit, for example.

The gate drive circuit 12A further includes a detector 12A4. Thedetector 12A4 is provided with a comparator 12A4 a and resistanceelements 12A4 b and 12A4 c connected in series. Voltage acrosscapacitors 12A5 a and 12A5 b connected in series is applied to thecomparator 12A4 a as an operating voltage. A divided voltage of theresistance elements 12A4 b and 12A4 c is input to a positive inputterminal of the comparator 12A4 a, and a divided voltage of theimpedance element group 16 described later is input to a negative inputterminal of the comparator 12A4 a.

Note that a power supply for applying the operating voltage to thedetector 12A4 can also be used as a power supply for driving the gatedrive circuit. A dedicated power supply for operating the detector 12A4need not be provided if the power supply therefor is also used as thepower supply for driving the gate drive circuit.

Provided outside the gate drive circuits 12A and 12B are the switchingsignal generation unit 20 that generates switching signals for drivingthe corresponding semiconductor switching elements 10A and 10B; theinsulating circuits 14A and 14B that receive the switching signalsgenerated by the switching signal generation unit 20 and transmit thesignals to the corresponding gate drive circuits 12A and 12B; and theimpedance element group 16 that detects the voltage between the secondmain terminal 10A2 of the semiconductor switching element 10A and thesecond main terminal 10B2 of the semiconductor switching element 10B.

The insulating circuit 14A is a circuit that electrically insulates theswitching signal generation unit 20 from the gate drive circuit 12A. Theinsulating circuit 14B is a circuit that electrically insulates theswitching signal generation unit 20 from the gate drive circuit 12B. Aphotocoupler can be used as each of the insulating circuits 14A and 14B.As for the insulating circuit 14A, the insulating circuit 14A iscomposed of a photocoupler including a light-emitting diode 14A1 and aphototransistor 14A2.

The impedance element group 16 is configured to include one or aplurality of impedance elements that are connected between theconnection point of the capacitors 12A5 a and 12A5 b that are connectedin series and serve as a power supply for the operation of the gatedrive circuit 12A, and the connection point of capacitors 12B5 a and12B5 b that are connected in series and serve as a power supply for theoperation of the gate drive circuit 12B. That is, FIG. 1 illustrates anexample in which the impedance element group 16 is disposed between thegate drive circuits 12A and 12B belonging to the same phase. FIG. 1illustrates five impedance elements 16 e 1, 16 e 2, 16 e 3, 16 e 4, and16 e 5 that are connected in series.

Note that the impedance element group 16 may be configured by connectingcapacitors or diodes in series instead of the impedance elements. Theimpedance element group 16 is not limited to the series connection ofthe impedance elements or capacitors, but may be configured byconnecting in series parallel circuits of the impedance elements orcapacitors. Alternatively, the impedance element group 16 may beconfigured by a combination of the impedance elements and capacitors.Note that although FIG. 1 illustrates the configuration that detects thevoltage across the impedance elements, the current flowing through theimpedance elements may be detected instead. For example, a photocouplermay be connected in series with the impedance elements so that thecurrent flowing through the impedance elements is detected by aphotodiode provided on a primary side of the photocoupler, and thedetected current is transmitted to the gate drive circuit via aphototransistor provided on a secondary side of the photocoupler.

Moreover, although FIG. 1 illustrates an example of detecting thedivided voltage generated in the impedance elements that make up theimpedance element group 16, the voltage need not be divided if an inputbreakdown voltage of each of the detectors 12A4 and 12B4 is high. Inthis case, the voltage across the impedance element group 16 may beapplied to the detectors 12A4 and 12B4 without being divided. Moreover,in this case, the impedance element group 16 may be configured toinclude a plurality of impedance elements connected in series or can beconfigured to include one impedance element.

Next, as an operation of the main part of the power converter accordingto the first embodiment, a description will be given of an operationperformed when driving the semiconductor switching element 10A making upthe first power semiconductor switching element of the power convertermain circuit 10.

The switching signal generation unit 20 generates a switching signal fordriving the semiconductor switching element 10A and outputs theswitching signal to the insulating circuit 14A.

When a command signal for controlling the semiconductor switchingelement 10A to be turned on (hereinafter referred to as an “on commandsignal”) is input to the insulating circuit 14A as the switching signalfrom the switching signal generation unit 20, for example, thelight-emitting diode 14A1 is illuminated to cause the phototransistor14A2 to become conductive. When a command signal for controlling thesemiconductor switching element 10A to be turned off (hereinafterreferred to as an “off command signal”) is input to the insulatingcircuit 14A as the switching signal from the switching signal generationunit 20, for example, the light-emitting diode 14A1 is turned off tocause the phototransistor 14A2 to become non-conductive. Accordingly,the on command signal and the off command signal from the switchingsignal generation unit 20 are recognized by the switching speed changingunit 12A3 of the gate drive circuit 12A as a change in the currentcaused by a change in the conduction state of the phototransistor 14A2.

Although the detailed operation of the detector 12A4 will be describedlater, the detector 12A4 can detect the voltage applied across thesemiconductor switching elements 10A and 10B by detecting the voltagebetween the gate drive circuits 12A and 12B. The detector 12A4 can alsodetect whether the voltage of the capacitor 6 (hereinafter referred toas a “capacitor voltage”) is higher or lower than a reference voltage bydetecting the voltage applied across the semiconductor switchingelements 10A and 10B. A detection signal by the detector 12A4 is inputto the switching speed changing unit 12A3.

The switching speed changing unit 12A3 changes the driving speed of thesemiconductor switching elements 10A and 10B on the basis of thedetection signal from the detector 12A4 and the command signal from theinsulating circuit 14A. Details of the operation in changing the drivingspeed of the semiconductor switching elements 10A and 10B are asfollows.

First, when the speed of turning on the semiconductor switching element10A is to be increased, the first on transistor 12A1 a and the second ontransistor 12A1 c are both controlled to be turned on while the firstoff transistor 12A1 b and the second off transistor 12A1 d are bothcontrolled to be turned off. With both the first on transistor 12A1 aand the second on transistor 12A1 c being turned on, the gate resistors12A2 a and 12A2 c are both connected in parallel to the first signalinput terminal 10A3, thereby reducing the gate resistance and increasingthe switching speed.

On the other hand, when the speed of turning on the semiconductorswitching element 10A is to be reduced, either one of the first ontransistor 12A1 a and the second on transistor 12A1 c is controlled tobe turned on while both the first off transistor 12A1 b and the secondoff transistor 12A1 d are controlled to be turned off. With only thefirst on transistor 12A1 a being turned on, for example, only the gateresistor 12A2 a is connected to the first signal input terminal 10A3,thereby increasing the gate resistance and reducing the switching speed.

When the speed of turning off the semiconductor switching element 10A isto be increased, the first on transistor 12A1 a and the second ontransistor 12A1 c are both controlled to be turned off while the firstoff transistor 12A1 b and the second off transistor 12A1 d are bothcontrolled to be turned on. With both the first off transistor 12A1 band the second off transistor 12A1 d being turned on, the gate resistors12A2 b and 12A2 d are both connected in parallel to the first signalinput terminal 10A3, thereby reducing the gate resistance and increasingthe switching speed.

When the speed of turning off the semiconductor switching element 10A isto be reduced, the first on transistor 12A1 a and the second ontransistor 12A1 c are both controlled to be turned off while either oneof the first off transistor 12A1 b and the second off transistor 12A1 dis controlled to be turned on. With only the first off transistor 12A1 bbeing turned on, for example, only the gate resistor 12A2 b is connectedto the first signal input terminal 10A3, thereby increasing the gateresistance and reducing the switching speed.

Note that the present invention is not limited to the aforementionedcontrols which are described as examples. For example, when the gateresistor 12A2 c having a resistance value smaller than the resistancevalue of the gate resistor 12A2 a is used, the first on transistor 12A1a connected to the gate resistor 12A2 a with a relatively largeresistance value may be controlled to be turned on in order to reducethe speed of turning on the semiconductor switching element 10A, or thesecond on transistor 12A1 c connected to the gate resistor 12A2 c with arelatively small resistance value may be controlled to be turned on inorder to increase the speed of turning on the semiconductor switchingelement 10A. For example, when the gate resistor 12A2 d having aresistance value smaller than the resistance value of the gate resistor12A2 b is used, the first off transistor 12A1 b connected to the gateresistor 12A2 b with a relatively large resistance value may becontrolled to be turned on in order to reduce the speed of turning offthe semiconductor switching element 10A, or the second off transistor12A1 d connected to the gate resistor 12A2 d with a relatively smallresistance value may be controlled to be turned on in order to increasethe speed of turning off the semiconductor switching element 10A.

The description of the operation provided above will be supplementedwith an operation in which the impedance element group 16 outputsinformation corresponding to the capacitor voltage (hereinafter referredto as “capacitor voltage information”).

The second main terminal 10A2 and the second signal input terminal 10A4of the semiconductor switching element 10A have the same potential. Thesecond signal input terminal 10A4 of the semiconductor switching element10A and one end of the impedance element group 16 are both connected tothe connection point of the capacitors 12A5 a and 12A5 b connected inseries, and thus have the same potential. These also apply to thesemiconductor switching element 10B. Accordingly, the voltage generatedacross the impedance element group 16 or the current flowing through theimpedance element group 16 represents a potential difference between thesecond main terminal 10A2 of the semiconductor switching element 10A andthe second main terminal 10B2 of the semiconductor switching element10B. The gate drive circuits 12A and 12B can thus know the potentialdifference between the second main terminal 10A2 of the semiconductorswitching element 10A and the second main terminal 10B2 of thesemiconductor switching element 10B by detecting the voltage or thecurrent across the impedance element group 16.

Note that, as one can see from FIG. 1, the impedance element group 16can be disposed near the gate drive circuits 12A and 12B. This canshorten a path, namely electrical wiring, through which the voltageinformation or current information from the impedance element group 16is transmitted to the gate drive circuits 12A and 12B, therebypreventing noise from being superimposed on the electrical wiring. As aresult, an effect is obtained where the gate drive circuits 12A and 12Bcan change or modify the method of driving the semiconductor switchingelements 10A and 10B with high accuracy by using accurate voltageinformation or accurate current information from the impedance elementgroup 16.

Next, the operation of the gate drive circuit will be described in moredetail with reference to FIGS. 2 and 3. FIGS. 2 and 3 are statetransition diagrams illustrating a change in the output voltage of thepower converter main circuit 10 according to the first embodiment, whereFIG. 2 is a state transition diagram when the load receives a current,or performs a sink operation, and FIG. 3 is a state transition diagramwhen the load discharges a current, or performs a source operation. Notethat FIGS. 2 and 3 omit display of the load. It is also assumed in thedescription that the voltage stored in the capacitor 6 is 1000 [V], anda negative terminal of the capacitor 6 is set as the referencepotential. That is, the potential at the negative terminal of thecapacitor 6 is 0 [V].

First, when the load performs a sink operation, the power converter maincircuit 10 repeatedly performs the operation of changing the state froma first state to a second state, from the second state to a third state,from the third state to a fourth state, and then from the fourth stateto the first state as illustrated in FIG. 2.

In the first state, the semiconductor switching element 10A is on, andthe semiconductor switching element 10B is off. At this point in time, acurrent flows from a positive terminal of the capacitor 6 to the loadthrough the semiconductor switching element 10A. The potential of theload matches the potential at the positive terminal of the capacitor 6.Thus, the potential at the second main terminal 10A2 of thesemiconductor switching element 10A is 1000 [V]. The potential at thesecond main terminal 10B2 of the semiconductor switching element 10B is0 [V]. A voltage of 1000 [V] identical to the capacitor voltage isapplied to the impedance element group 16.

The power converter main circuit 10 transitions from the first state tothe second state when the semiconductor switching element 10A isswitched from on to off. This causes the potential of each part tochange. The current path from the capacitor 6 to the load changes aswell.

In the second state, the semiconductor switching element 10A is off, andthe semiconductor switching element 10B is off. At this point in time, acurrent flows from the negative terminal of the capacitor 6 to the loadthrough the semiconductor switching element 10B. The potential of theload matches the potential at the negative terminal of the capacitor 6.Thus, the potential at the second main terminal 10A2 of thesemiconductor switching element 10A is 0 [V], and the potential at thesecond main terminal 10B2 of the semiconductor switching element 10B isalso 0 [V]. Moreover, a voltage of 0 [V] is applied to the impedanceelement group 16.

The power converter main circuit 10 transitions from the second state tothe third state when the semiconductor switching element 10B is switchedfrom off to on. However, there is no change in the potential of eachpart. The current path from the capacitor 6 to the load does not change,either.

In the third state, the semiconductor switching element 10A is off, andthe semiconductor switching element 10B is on. At this point in time, acurrent flows from the negative terminal of the capacitor 6 to the loadthrough the semiconductor switching element 10B. The potential of theload matches the potential at the negative terminal of the capacitor 6.Thus, the potential at the second main terminal 10A2 of thesemiconductor switching element 10A is 0 [V], and the potential at thesecond main terminal 10B2 of the semiconductor switching element 10B isalso 0 [V]. Moreover, a voltage of 0 [V] is applied to the impedanceelement group 16.

The power converter main circuit 10 transitions from the third state tothe fourth state when the semiconductor switching element 10B isswitched from on to off. However, there is no change in the potential ofeach part. The current path from the capacitor 6 to the load does notchange, either.

In the fourth state, the semiconductor switching element 10A is off, andthe semiconductor switching element 10B is off. At this point in time, acurrent flows from the negative terminal of the capacitor 6 to the loadthrough the semiconductor switching element 10B. The potential of theload matches the potential at the negative terminal of the capacitor 6.Thus, the potential at the second main terminal 10A2 of thesemiconductor switching element 10A is 0 [V], and the potential at thesecond main terminal 10B2 of the semiconductor switching element 10B isalso 0 [V]. Moreover, a voltage of 0 [V] is applied to the impedanceelement group 16.

The power converter main circuit 10 transitions from the fourth state tothe first state when the semiconductor switching element 10A is switchedfrom off to on. This causes the potential of each part to change. Thecurrent path from the capacitor 6 to the load changes as well.

In the first state, the semiconductor switching element 10A is on, andthe semiconductor switching element 10B is off. At this point in time, acurrent flows from the positive terminal of the capacitor 6 to the loadthrough the semiconductor switching element 10A. The potential of theload matches the potential at the positive terminal of the capacitor 6.Thus, the potential at the second main terminal 10A2 of thesemiconductor switching element 10A is 1000 [V]. The potential at thesecond main terminal 10B2 of the semiconductor switching element 10B is0 [V]. Moreover, a voltage of 1000 [V] identical to the capacitorvoltage is applied to the impedance element group 16.

FIG. 3 will now be referenced. When the load performs a sourceoperation, the power converter main circuit 10 repeatedly performs theoperation of changing the state from a fifth state to a sixth state,from the sixth state to a seventh state, from the seventh state to aneighth state, and from the eighth state to the fifth state asillustrated in FIG. 3.

In the fifth state, the semiconductor switching element 10A is on, andthe semiconductor switching element 10B is off. At this point in time, acurrent flows from the load to the positive terminal of the capacitor 6through the semiconductor switching element 10A. The potential of theload matches the potential at the positive terminal of the capacitor 6.The potential at the second main terminal 10A2 of the semiconductorswitching element 10A is 1000 [V]. The potential at the second mainterminal 10B2 of the semiconductor switching element 10B is 0 [V]. Avoltage of 1000 [V] identical to the capacitor voltage is applied to theimpedance element group 16.

The power converter main circuit 10 transitions from the fifth state tothe sixth state when the semiconductor switching element 10A is switchedfrom on to off. However, there is no change in the potential of eachpart. The current path from the load to the capacitor 6 does not change,either.

In the sixth state, the semiconductor switching element 10A is off, andthe semiconductor switching element 10B is off. A current flows from theload to the positive terminal of the capacitor 6 through thesemiconductor switching element 10A. The potential of the load matchesthe potential at the positive terminal of the capacitor 6. Thus, thepotential at the second main terminal 10A2 of the semiconductorswitching element 10A is 1000 [V], and the potential at the second mainterminal 10B2 of the semiconductor switching element 10B is 0 [V].Moreover, a voltage of 1000 [V] identical to the capacitor voltage isapplied to the impedance element group 16.

The power converter main circuit 10 transitions from the sixth state tothe seventh state when the semiconductor switching element 10B isswitched from off to on. This causes the potential of each part tochange. The current path from the load to the capacitor 6 changes aswell.

In the seventh state, the semiconductor switching element 10A is off,and the semiconductor switching element 10B is on. At this point intime, a current flows from the load to the negative terminal of thecapacitor 6 through the semiconductor switching element 10B. Thepotential of the load matches the potential at the negative terminal ofthe capacitor 6. Thus, the potential at the second main terminal 10A2 ofthe semiconductor switching element 10A is 0 [V], and the potential atthe second main terminal 10B2 of the semiconductor switching element 10Bis also 0 [V]. Moreover, a voltage of 0 [V] is applied to the impedanceelement group 16.

The power converter main circuit 10 transitions from the seventh stateto the eighth state when the semiconductor switching element 10B isswitched from on to off. This causes the potential of each part tochange. The current path from the load to the capacitor 6 changes aswell.

In the eighth state, the semiconductor switching element 10A is off, andthe semiconductor switching element 10B is off. At this point in time, acurrent flows from the load to the positive terminal of the capacitor 6through the semiconductor switching element 10A. The potential of theload matches the potential at the positive terminal of the capacitor 6.Thus, the potential at the second main terminal 10A2 of thesemiconductor switching element 10A is 1000 [V]. The potential at thesecond main terminal 10B2 of the semiconductor switching element 10B is0 [V]. A voltage of 1000 [V] identical to the capacitor voltage isapplied to the impedance element group 16.

The power converter main circuit 10 transitions from the eighth state tothe fifth state when the semiconductor switching element 10A is switchedfrom off to on. However, there is no change in the potential of eachpart. The current path from the load to the capacitor 6 does not change,either.

In the fifth state, the semiconductor switching element 10A is on, andthe semiconductor switching element 10B is off. At this point in time, acurrent flows from the load to the positive terminal of the capacitor 6through the semiconductor switching element 10A. The potential of theload matches the potential at the positive terminal of the capacitor 6.Thus, the potential at the second main terminal 10A2 of thesemiconductor switching element 10A is 1000 [V]. The potential at thesecond main terminal 10B2 of the semiconductor switching element 10B is0 [V]. Moreover, a voltage of 1000 [V] identical to the capacitorvoltage is applied to the impedance element group 16.

Next, the operation of the power converter with a focus on the state ofthe power converter main circuit will be described with reference toFIGS. 1 to 3 as appropriate.

First, the description focuses on the gate drive circuit 12A at the timeof transition from the first state to the second state. As for thedrawings, reference is made to FIGS. 1 and 2.

In the first state, a voltage of 1000 [V] identical to the capacitorvoltage is applied to the impedance element group 16. The detector 12A4included in the gate drive circuit 12A detects the voltage across theimpedance element group 16 and transmits information on the detectedcapacitor voltage, namely the capacitor voltage information, to theswitching speed changing unit 12A3. The switching speed changing unit12A3 included in the gate drive circuit 12A can know the capacitorvoltage from the voltage between the gate drive circuit 12A and the gatedrive circuit 12B.

When determining that the voltage between the gate drive circuit 12A andthe gate drive circuit 12B is lower than the reference voltage, theswitching speed changing unit 12A3 performs control to increase thedriving speed for turning off the semiconductor switching element 10A.This control can reduce or prevent a switching loss at turn-off. Notethat although the control for increasing the driving speed increases thesurge voltage generated at turn-off, the capacitor voltage is low, sothat no excessive voltage is applied to the components inside the powerconverter main circuit 10. The power converter main circuit 10 thustransitions from the first state to the second state.

Next, the description focuses on the gate drive circuit 12B at the timeof transition from the sixth state to the seventh state. As for thedrawings, reference is made to FIGS. 1 and 3. In the sixth state, avoltage of 1000 [V] identical to the capacitor voltage is applied to theimpedance element group 16. The detector 12B4 included in the gate drivecircuit 12B detects the voltage across the impedance element group 16and transmits the detected capacitor voltage information to theswitching speed changing unit 12B3. The switching speed changing unit12B3 included in the gate drive circuit 12B can know the capacitorvoltage from the voltage between the gate drive circuit 12B and the gatedrive circuit 12A.

When determining that the voltage between the gate drive circuit 12B andthe gate drive circuit 12A is lower than the reference voltage, theswitching speed changing unit 12B3 performs control to increase thedriving speed for turning on the semiconductor switching element 10B.This control can reduce or prevent a switching loss at turn-on. Notethat although the surge voltage generated at turn-on is increased, thecapacitor voltage is low, so that no excessive voltage is applied to thecomponents inside the power converter main circuit 10. The powerconverter main circuit 10 thus transitions from the sixth state to theseventh state.

Note that although FIG. 1 illustrates the configuration in which thedetectors 12A4 and 12B4 are respectively provided in the gate drivecircuits 12A and 12B, the detector may be provided in only one of thegate drive circuits 12A and 12B. For example, FIG. 4 illustrates aconfiguration in which the detector 12A4 is provided only in the gatedrive circuit 12A, where the configuration as illustrated in FIG. 4 canalso obtain the effect described above.

According to the power converter of the first embodiment describedabove, one or a plurality of the impedance elements are connectedbetween at least a pair of the gate drive circuits among the gate drivecircuits driving corresponding ones of two or more of the semiconductorswitching elements included in the power converter main circuit, atleast one of the gate drive circuits is provided with the detectordetecting the voltage across the impedance elements or the currentflowing therethrough, and the output of the detector is transmitted tothe gate drive circuit, whereby the influence of noise can be reducedwhen the voltage information on each part of the power converter istransmitted to the gate drive circuit.

Moreover, according to the power converter of the first embodiment, thedriving speed of the semiconductor switching element is changed inaccordance with the output of the detector indicating the capacitorvoltage, whereby generation of an excessive voltage in the componentsinside the power converter main circuit can be prevented.

Second Embodiment

FIG. 5 is a circuit diagram illustrating the configuration of a mainpart of a power converter according to a second embodiment. The powerconverter according to the second embodiment illustrated in FIG. 5discloses a configuration in which storages 12A5 and 12B5 are includedin the gate drive circuits 12A and 12B, respectively, in theconfiguration of the power converter according to the first embodimentillustrated in FIG. 1. According to FIG. 5, the storage 12A5 is addedbetween the detector 12A4 and the switching speed changing unit 12A3,and the storage 12B5 is added between the detector 12B4 and theswitching speed changing unit 12B3. Note that the other components areidentical or equivalent to those in FIG. 1 and are thus denoted by thesame reference numerals as those in FIG. 1, whereby a redundantdescription will be omitted.

The detector 12A4 provided in the gate drive circuit 12A in FIG. 5determines the magnitude relationship between the voltage across theimpedance element group 16 representing the capacitor voltageinformation and the reference voltage, and transmits the result of thedetermination to the storage 12A5. The storage 12A5 stores determinationinformation as to whether the capacitor voltage is lower or higher thanthe reference voltage. The storage 12A5 may be any means capable ofstoring the capacitor voltage information, and can include a voltagestorage element, a latch circuit, or the like.

The storage 12A5 acts on the switching speed changing unit 12A3, whichchanges the driving speed of the semiconductor switching element 10A onthe basis of the determination information stored in the storage 12A5.

Note that an operating power supply for operating the detector 12A4 andthe storage 12A5 can also be used as a power supply for driving the gatedrive circuit. A dedicated power supply for operating the detector 12A4and the storage 12A5 need not be provided if the power supply thereforis also used as the power supply for driving the gate drive circuit.

Next, the operation of the main part of the power converter according tothe second embodiment will be described while focusing on the operationof the gate drive circuit 12A in the first state of FIG. 2. As for thedrawings, reference is made to FIGS. 2 and 5.

In the first state, a voltage of 1000 [V] identical to the capacitorvoltage is applied to the impedance element group 16. The detector 12A4determines the magnitude relationship between the voltage from theimpedance element group 16 and the reference voltage. When the detector12A4 determines that the voltage from the impedance element group 16 islower than the reference voltage, the result of the determination isstored in the storage 12A5.

The switching speed changing unit 12A3 switches control to one thatincreases the driving speed of the semiconductor switching element 10Aon the basis of the information determined to be lower than thereference voltage and stored in the storage 12A5. This control preventsa turn-off switching loss of the semiconductor switching element 10A atthe time of transition from the first state to the second state. Aturn-on switching loss of the semiconductor switching element 10A canalso be prevented at the time of transition from the fourth state to thefirst state. Note that although the above control increases the surgevoltage generated at the time of switching, the capacitor voltage islow, so that no excessive voltage is applied to the components insidethe power converter main circuit 10.

On the other hand, when the detector 12A4 determines that the voltagefrom the impedance element group 16 is higher than the reference voltagein the first state, the result of the determination is stored in thestorage 12A5. Information stored in the storage 12A5 is overwritten witha new result of the determination. That is, the latest result of thedetermination is stored in the storage 12A5.

The switching speed changing unit 12A3 switches control to one thatreduces the driving speed of the semiconductor switching element 10A onthe basis of the information determined to be higher than the referencevoltage and stored in the storage 12A5. Although the capacitor voltageis high, this control reduces the surge voltage generated at the time ofswitching to be able to prevent application of an excessive voltage tothe components inside the power converter main circuit 10.

Next, the description focuses on the operation of the gate drive circuit12B in the fifth, sixth, and eighth states of FIG. 3. As for thedrawings, reference is made to FIGS. 3 and 5.

In each of the fifth, sixth, and eighth states, a voltage of 1000 [V]identical to the capacitor voltage is applied to the impedance elementgroup 16. The detector 12B4 determines the magnitude relationshipbetween the voltage from the impedance element group 16 and thereference voltage. When the detector 12B4 determines that the voltagefrom the impedance element group 16 is lower than the reference voltage,the result of the determination is stored in the storage 12B5.

The switching speed changing unit 12B3 switches control to one thatincreases the driving speed of the semiconductor switching element 10Bon the basis of the information determined to be lower than thereference voltage and stored in the storage 12B5. This control preventsa turn-off switching loss of the semiconductor switching element 10B atthe time of transition from the sixth state to the seventh state. Aturn-off switching loss of the semiconductor switching element 10B canalso be prevented at the time of transition from the seventh state tothe eighth state. Note that although the above control increases thesurge voltage generated at the time of switching, the capacitor voltageis low, so that no excessive voltage is applied to the components insidethe power converter main circuit 10.

On the other hand, when the detector 12B4 determines that the voltagefrom the impedance element group 16 is higher than the reference voltagein each of the fifth, sixth, and eighth states, the result of thedetermination is stored in the storage 12B5. Information stored in thestorage 12B5 is overwritten with a new result of the determination. Thatis, the latest result of the determination is stored in the storage12B5.

The switching speed changing unit 12B3 switches control to one thatreduces the driving speed of the semiconductor switching element 10B onthe basis of the information determined to be higher than the referencevoltage and stored in the storage 12B5. Although the capacitor voltageis high, this control reduces the surge voltage generated at the time ofswitching to be able to prevent application of an excessive voltage tothe components inside the power converter main circuit 10.

Note that although FIG. 5 illustrates the configuration in which thestorages 12A5 and 12B5 are respectively provided in the gate drivecircuits 12A and 12B, the aforementioned effect can also be obtained byprovision of the storage in only one of the gate drive circuits 12A and12B. When one of the storages 12A5 and 12B5 is omitted, the detectorcorresponding to the omitted detector 12A4 or 12B4 can be omitted inaccordance with the omission of the storage.

Third Embodiment

FIG. 6 is a circuit diagram illustrating the configuration of a mainpart of a power converter according to a third embodiment. As theconfiguration of the main part of the power converter according to thethird embodiment, FIG. 6 illustrates the load 2 to be driven; the powerconverter main circuit 10 that drives the load 2; gate drive circuits12A, 12B, 12C, and 12D that are peripheral circuits that control thepower converter main circuit 10; the impedance element group 16outputting the capacitor voltage information; and capacitors 6A and 6B,each of which is a power supply source for the power converter maincircuit 10 and accumulates DC power.

In the power converter according to the third embodiment, the twocapacitors 6A and 6B for accumulating DC voltage are connected in seriesto provide three terminals that are the upper DC terminal 8A, anintermediate DC terminal 8C, and the lower DC terminal 8B in thedescending order from one having a higher potential. The DC bus 7A onthe high potential side is electrically connected to the upper DCterminal 8A, and the DC bus 7B on the low potential side is electricallyconnected to the lower DC terminal 8B.

From the DC bus 7A on the high potential side to the DC bus 7B on thelow potential side, the power converter main circuit 10 according to thethird embodiment includes the semiconductor switching element 10A as thefirst power semiconductor switching element; the semiconductor switchingelement 10B as the second power semiconductor switching element; asemiconductor switching element 10C as a third power semiconductorswitching element; and a semiconductor switching element 10D as a fourthpower semiconductor switching element that are connected in series inthis order. The power converter main circuit 10 according to the thirdembodiment is further provided with a clamping diode 11A as a firstdiode element, a cathode of which is electrically connected to aconnection point of the semiconductor switching element 10A and thesemiconductor switching element 10B while an anode of which iselectrically connected to the intermediate DC terminal 8C; and aclamping diode 11B as a second diode element, a cathode of which iselectrically connected to the intermediate DC terminal 8C while an anodeof which is electrically connected to a connection point of thesemiconductor switching element 10C and the semiconductor switchingelement 10D. Note that the load 2 is connected to the electricalconnection point of the semiconductor switching element 10B and thesemiconductor switching element 10C.

The semiconductor switching element 10A is provided with the first mainterminal 10A1, the second main terminal 10A2, the first signal inputterminal 10A3, and the second signal input terminal 10A4. The sameapplies to the semiconductor switching elements 10B, 10C, and 10D, wherethe semiconductor switching element 10B is provided with the first mainterminal 10B1, the second main terminal 10B2, the first signal inputterminal 10B3, and the second signal input terminal 10B4, thesemiconductor switching element 10C is provided with a first mainterminal 10C1, a second main terminal 10C2, a first signal inputterminal 10C3, and a second signal input terminal 10C4, and thesemiconductor switching element 10D is provided with a first mainterminal 10D1, a second main terminal 10D2, a first signal inputterminal 10D3, and a second signal input terminal 10D4.

In the power converter main circuit 10 connected as described above, thepotential of the upper DC terminal 8A is applied to the load 2 when thesemiconductor switching elements 10A and 10B become conductive, and thepotential of the lower DC terminal 8B is applied to the load 2 when thesemiconductor switching elements 10C and 10D become conductive. Thepotential of the intermediate DC terminal 8C is applied to the load 2when either one of the semiconductor switching elements 10B and 10Cbecomes conductive while the semiconductor switching elements 10A and10D are non-conductive. The power converter main circuit 10 thus outputsthree kinds of potentials that are the potential of the upper DCterminal 8A, the potential of the intermediate DC terminal 8C, and thepotential of the lower DC terminal 8B, thereby operating as athree-level power converter circuit.

When AC power is supplied to the load 2, the semiconductor switchingelements 10A and 10B and the semiconductor switching elements 10C and10D operate symmetrically, and the two capacitors 6A and 6B store equalDC voltage.

Although FIG. 6 illustrates a MOSFET as the transistor element, thetransistor element is not limited to the MOSFET but may be any devicethat can switch a state between a low resistance state and a highresistance state by an electrical signal. For example, an IGBT or abipolar transistor may be used as the transistor element. Moreover, awide band-gap semiconductor such as SiC, GaN, or diamond in addition toSi which is used widely may be used as a material of the transistorelement and the diode element making up each of the semiconductorswitching elements 10A to 10D.

The gate drive circuit 12A is the first gate drive circuit that drivesthe semiconductor switching element 10A that is the first powersemiconductor switching element. Likewise, the gate drive circuit 12B isthe second gate drive circuit that drives the semiconductor switchingelement 10B that is the second power semiconductor switching element,the gate drive circuit 12C is a third gate drive circuit that drives thesemiconductor switching element 10C that is the third powersemiconductor switching element, and the gate drive circuit 12D is afourth gate drive circuit that drives the semiconductor switchingelement 10D that is the fourth power semiconductor switching element.

Here, among the gate drive circuits 12A to 12D illustrated in FIG. 6,the configuration of the gate drive circuit 12C is identical orequivalent to the configuration of the gate drive circuit 12A accordingto the first embodiment illustrated in FIG. 1. On the other hand, theconfiguration of each of the gate drive circuits 12B and 12D omits thedetector 12A4 from the configuration of the gate drive circuit 12Aillustrated in FIG. 1, and is identical or equivalent to theconfiguration of the gate drive circuit 12B illustrated in FIG. 4. Notethat in FIG. 6, a component identical or equivalent to a componentillustrated in FIG. 1 is denoted by the same reference numeral as thatassigned to the component in FIG. 1. Moreover, FIG. 6 omits illustrationof components corresponding to the insulating circuits 14A and 14B andthe switching signal generation unit 20 which are illustrated in FIG. 1.

As with the first embodiment, the voltage generated across the impedanceelement group 16 or the current flowing therethrough represents apotential difference between the second main terminal 10C2 of thesemiconductor switching element 10C and the second main terminal 10D2 ofthe semiconductor switching element 10D. The gate drive circuit 12C canthus know the potential difference between the second main terminal 10C2of the semiconductor switching element 10C and the second main terminal10D2 of the semiconductor switching element 10D by detecting the voltageor the current across the impedance element group 16.

Next, the operation of the power converter according to the thirdembodiment will be described. Note that the operation of the gate drivecircuit will be described with reference to the gate drive circuit 12Cthat includes a switching speed changing unit 12C3 and a detector 12C4.The potential of the intermediate DC terminal 8C is set to the referencepotential (0 [V]), and the voltage stored in each of the capacitors 6Aand 6B is set to 1000 [V]. Accordingly, the potential of the upper DCterminal 8A is +1000 [V], the potential of the intermediate DC terminal8C is 0 [V], and the potential of the lower DC terminal 8B is −1000 [V].

FIG. 7 is a circuit diagram illustrating one mode of the operation ofthe power converter according to the third embodiment. FIG. 7illustrates the mode in which the semiconductor switching elements 10Aand 10B are on, the semiconductor switching elements 10C and 10D areoff, and the potential of the load (not illustrated) matches thepotential of the upper DC terminal 8A. At this point in time, a currentflows between the upper DC terminal 8A and the load via thesemiconductor switching elements 10A and 10B. Thus, the potential at thesecond main terminal 10A2 of the semiconductor switching element 10A is+1000 [V], and the potential at the second main terminal 10B2 of thesemiconductor switching element 10B is also +1000 [V]. The potential atthe second main terminal 10C2 of the semiconductor switching element 10Cis 0 [V], and the potential at the second main terminal 10D2 of thesemiconductor switching element 10D is −1000 [V].

FIG. 8 is a circuit diagram illustrating one mode of the operation ofthe power converter according to the third embodiment, the mode beingdifferent from that of FIG. 7. FIG. 8 illustrates the mode in which thesemiconductor switching elements 10A and 10D are off, the semiconductorswitching elements 10B and 10C are on, and the potential of the loadmatches the potential of the intermediate DC terminal 8C. At this pointin time, a current flows between the intermediate DC terminal 8C and theload via the semiconductor switching elements 10B and 10C. Thus, thepotential at the second main terminal 10A2 of the semiconductorswitching element 10A is 0 [V], the potential at the second mainterminal 10B2 of the semiconductor switching element 10B is 0 [V], andthe potential at the second main terminal 10C2 of the semiconductorswitching element 10C is also 0 [V]. On the other hand, the potential atthe second main terminal 10D2 of the semiconductor switching element 10Dis −1000 [V].

FIG. 9 is a circuit diagram illustrating one mode of the operation ofthe power converter according to the third embodiment, the mode beingdifferent from that of each of FIGS. 7 and 8. FIG. 9 illustrates themode in which the semiconductor switching elements 10A and 10B are off,the semiconductor switching elements 10C and 10D are on, and thepotential of the load matches the potential of the lower DC terminal 8B.At this point in time, a current flows between the lower DC terminal 8Band the load via the semiconductor switching elements 10C and 10D. Thus,the potential at the second main terminal 10A2 of the semiconductorswitching element 10A is 0 [V]. On the other hand, the potential at thesecond main terminal 10B2 of the semiconductor switching element 10B is−1000 [V], the potential at the second main terminal 10C2 of thesemiconductor switching element 10C is −1000 [V], and the potential atthe second main terminal 10D2 of the semiconductor switching element 10Dis also −1000 [V].

The switching operation of the semiconductor switching elements 10A to10D generates a surge voltage, which is known to be particularly largewhen the following two operations are performed in the three-levelcircuit.

A first operation refers to a case where the load receives a current,namely performs a sink operation, and is an operation in which thesemiconductor switching element 10B that is the second powersemiconductor switching element performs a switching operation to causeswitching of the state between the state in FIG. 8 and the state in FIG.9. A second operation refers to a case where the load discharges acurrent, namely performs a source operation, and is an operation inwhich the semiconductor switching element 10C that is the third powersemiconductor switching element performs a switching operation to causeswitching of the state between the state in FIG. 7 and the state in FIG.8. These operations involve a particularly large change in the currentpath and are thus known to cause generation of a particularly largesurge voltage.

The description will focus on the operation in which the semiconductorswitching element 10C that is the third power semiconductor switchingelement performs a switching operation to cause switching of the statebetween the state in FIG. 7 and the state in FIG. 8 when the loaddischarges a current.

With the configuration illustrated in FIG. 6, a voltage of 1000 [V]identical to the capacitor voltage is applied across the impedanceelement group 16 in the states of FIGS. 7 and 8. The detector 12C4included in the third gate drive circuit 12C detects the voltage acrossthe impedance element group 16. The gate drive circuit 12C can thus knowthe voltage between the gate drive circuit 12C and the gate drivecircuit 12D, or the capacitor voltage.

When determining that the voltage between the third gate drive circuit12C and the fourth gate drive circuit 12D is lower than the referencevoltage, the third gate drive circuit 12C switches control to one thatincreases the driving speed for turning on and off the semiconductorswitching element 10C. The increase in the driving speed can prevent aturn-on switching loss and a turn-off switching loss. Although the surgevoltage generated at the time of switching increases, the capacitorvoltage is low, so that no excessive voltage is applied to thecomponents inside the power converter main circuit 10.

On the other hand, when determining that the voltage between the thirdgate drive circuit 12C and the fourth gate drive circuit 12D is higherthan the reference voltage, the third gate drive circuit 12C switchescontrol to one that reduces the driving speed for turning on and off thesemiconductor switching element 10C. Although the capacitor voltage ishigh, the decrease in the driving speed reduces the surge voltagegenerated at the time of switching of turn-on and turn-off to be able toprevent application of an excessive voltage to the components inside thepower converter main circuit 10.

FIG. 10 is a circuit diagram illustrating the configuration of a mainpart of the power converter according to the third embodiment, theconfiguration being different from that of FIG. 6. The characteristic ofthe configuration illustrated in FIG. 10 is that the impedance elementgroup 16 is connected between the gate drive circuit 12A that is thefirst gate drive circuit and the gate drive circuit 12B that is thesecond gate drive circuit, and that the gate drive circuit 12B that isthe second gate drive circuit includes the detector 12B4 and the storage12B5. Note that the storage 12B5 can be omitted.

Next, the operation of the main part of the power converter illustratedin FIG. 10 will be described while focusing on the operation of thesemiconductor switching element 10B that is the second powersemiconductor switching element when the load receives a current, orperforms a sink operation. As for the drawings, reference is made toFIGS. 8 and 9.

In the state illustrated in FIG. 9, a voltage of 1000 [V] identical tothe capacitor voltage is applied to the impedance element group 16. Thedetector 12B4 included in the gate drive circuit 12B determines themagnitude relationship between the voltage across the impedance elementgroup 16 representing the capacitor voltage information and thereference voltage, and transmits the result of the determination to thestorage 12B5. The storage 12B5 stores determination information as towhether the capacitor voltage is lower or higher than the referencevoltage.

The storage 12B5 acts on the switching speed changing unit 12B3, whichchanges the driving speed of the semiconductor switching element 10B onthe basis of the determination information stored in the storage 12B5.

When the detector 12B4 determines that the voltage from the impedanceelement group 16 is lower than the reference voltage, the result of thedetermination is stored in the storage 12B5.

The switching speed changing unit 12B3 switches control to one thatincreases the driving speed of the semiconductor switching element 10Bon the basis of the information determined to be lower than thereference voltage and stored in the storage 12B5. This control preventsa switching loss of the semiconductor switching element 10B. Note thatalthough the above control increases the surge voltage generated at thetime of switching, the capacitor voltage is low, so that no excessivevoltage is applied to the components inside the power converter maincircuit 10.

On the other hand, when the detector 12B4 determines that the voltagefrom the impedance element group 16 is higher than the referencevoltage, the result of the determination is stored in the storage 12B5.Information stored in the storage 12B5 is overwritten with a new resultof the determination. That is, the latest result of the determination isstored in the storage 12B5.

The switching speed changing unit 12B3 switches control to one thatreduces the driving speed of the semiconductor switching element 10B onthe basis of the information determined to be higher than the referencevoltage and stored in the storage 12B5. Although the capacitor voltageis high, this control reduces the surge voltage generated at the time ofswitching to be able to prevent application of an excessive voltage tothe components inside the power converter main circuit 10.

FIG. 11 is a circuit diagram illustrating the configuration of a mainpart of the power converter according to the third embodiment, theconfiguration being different from that of each of FIGS. 6 and 10. Thecharacteristic of the configuration illustrated in FIG. 11 is that animpedance element group 18 is connected between the gate drive circuit12A that is the first gate drive circuit and the gate drive circuit 12Dthat is the fourth gate drive circuit, the gate drive circuit 12A thatis the first gate drive circuit includes the detector 12A4, and the gatedrive circuit 12D that is the fourth gate drive circuit includes adetector 12D4. Note that a storage can also be included as in FIG. 10.

With the configuration illustrated in FIG. 11, a voltage of 2000 [V]corresponding to the voltage of the two capacitors 6A and 6B is appliedacross the impedance element group 18 in the state of FIG. 7. For thisreason, the impedance element group 18 in the configuration illustratedin FIG. 11 includes impedance elements twice as many as the case of FIG.6, or includes ten impedance elements 18 e 1 to 18 e 10. Note that inthe state illustrated in each of FIGS. 8 and 9, a voltage of 1000 [V]corresponding to the voltage of one capacitor is applied.

The detectors 12A4 and 12D4 included in the gate drive circuits 12A and12D, respectively, can know the capacitor voltage on the basis ofvoltage information or current information output from the impedanceelement group 18. The switching speed changing unit 12A3 provided in thegate drive circuit 12A performs control to switch or change the drivingspeed of the semiconductor switching element 10A, and a switching speedchanging unit 12D3 provided in the gate drive circuit 12D performscontrol to switch or change the driving speed of the semiconductorswitching element 10D.

When determining that the capacitor voltage is low, the gate drivecircuit 12A switches control to one that increases the driving speed forturning on and off the semiconductor switching element 10A. Similarly,when determining that the capacitor voltage is low, the gate drivecircuit 12D switches control to one that increases the driving speed forturning on and off the semiconductor switching element 10D. The increasein the driving speed can prevent a turn-on switching loss and a turn-offswitching loss. Note that although the above control increases the surgevoltage generated at the time of switching, the capacitor voltage islow, so that no excessive voltage is applied to the components insidethe power converter main circuit 10.

As described above, the third embodiment describes the configuration asillustrated in FIGS. 6, 10, and 11 in which a pair of two gate drivecircuits is selected from the plurality of gate drive circuits and thetwo gate drive circuits of the pair are connected by the impedanceelement group 16 or 18. However, the present invention is not limited tothese configurations. A plurality of pairs of two gate drive circuitsmay be selected from the plurality of gate drive circuits and the twogate drive circuits of each pair may be connected by the impedanceelement group. For example, the configuration as illustrated in FIG. 12may be adopted. The configuration illustrated in FIG. 12 is acombination of the configurations in FIGS. 6, 10, and 11, in which animpedance element group 16A including the impedance elements 16 e 1 to16 e 5 is disposed between the gate drive circuits 12A and 12B, animpedance element group 16B including impedance elements 16 e 6 to 16 e10 is disposed between the gate drive circuits 12C and 12D, and theimpedance element group 18 including the impedance elements 18 e 1 to 18e 10 is disposed between the gate drive circuits 12A and 12D.

Note that as one can see from the configuration in FIG. 10, for example,the impedance element group 16 can be disposed near the gate drivecircuits 12A and 12B. This can shorten a path, namely electrical wiring,through which the voltage information or current information from theimpedance element group 16 is transmitted to the gate drive circuits 12Aand 12B, thereby preventing noise from being superimposed on theelectrical wiring. As a result, an effect is obtained where the gatedrive circuits 12A and 12B can change the method of driving thesemiconductor switching elements 10A and 10B with high accuracy by usingaccurate voltage information or accurate current information from theimpedance element group 16.

Fourth Embodiment

FIG. 13 is a circuit diagram illustrating the configuration of a mainpart of a power converter according to a fourth embodiment. As theconfiguration of the main part of the power converter according to thefourth embodiment, FIG. 13 illustrates the load 2 to be driven, thepower converter main circuit 10 that drives the load 2; gate drivecircuits 12UA, 12UB, 12VA, and 12VB that are peripheral circuits thatcontrol the power converter main circuit 10; the impedance element group16 outputting the capacitor voltage information; and the capacitor 6that is a power supply source for the power converter main circuit 10and accumulates DC power.

The power converter main circuit 10 of the fourth embodiment isconfigured to include a semiconductor switching element 10UA as a firstU-phase power semiconductor switching element connected to the DC bus 7Aon the high potential side; a semiconductor switching element 10UB as asecond U-phase power semiconductor switching element connected to the DCbus 7B on the low potential side; a semiconductor switching element 10VAas a first V-phase power semiconductor switching element connected tothe DC bus 7A on the high potential side; and a semiconductor switchingelement 10VB as a second V-phase power semiconductor switching elementconnected to the DC bus 7B on the low potential side. The semiconductorswitching elements 10UA and 10UB are connected in series, and one end ofthe load 2 is connected to an output terminal 5U that is the electricalconnection point of the elements. The semiconductor switching elements10VA and 10VB are connected in series, and another end of the load 2 isconnected to an output terminal 5V that is the electrical connectionpoint of the elements. The power converter main circuit 10 thus forms asingle-phase inverter in which the U-phase semiconductor switchingelements 10UA and 10UB and the V-phase semiconductor switching elements10VA and 10VB are bridge-connected.

The semiconductor switching element 10UA is provided with a first mainterminal 10UA1, a second main terminal 10UA2, a first signal inputterminal 10UA3, and a second signal input terminal 10UA4, while thesemiconductor switching element 10UB is provided with a first mainterminal 10UB1, a second main terminal 10UB2, a first signal inputterminal 10UB3, and a second signal input terminal 10UB4. Similarly, thesemiconductor switching element 10VA is provided with a first mainterminal 10VA1, a second main terminal 10VA2, a first signal inputterminal 10VA3, and a second signal input terminal 10VA4, while thesemiconductor switching element 10VB is provided with a first mainterminal 10VB1, a second main terminal 10VB2, a first signal inputterminal 10VB3, and a second signal input terminal 10VB4.

In the power converter main circuit 10 configured as described above,the first main terminal 10UA1 of the semiconductor switching element10UA is connected to the DC bus 7A, and the second main terminal 10UB2of the semiconductor switching element 10UB is connected to the DC bus7B. Similarly, the first main terminal 10VA1 of the semiconductorswitching element 10VA is connected to the DC bus 7A, and the secondmain terminal 10VB2 of the semiconductor switching element 10VB isconnected to the DC bus 7B.

The DC bus 7A is connected to the upper DC terminal 8A of the capacitor6, and the DC bus 7B is connected to the lower DC terminal 8B of thecapacitor 6. That is, the voltage of the capacitor 6 is applied betweenthe DC buses 7A and 7B.

In the power converter main circuit 10 connected as described above, thepotential of the DC bus 7A is applied to the load 2 when thesemiconductor switching element 10UA or 10VA becomes conductive, and thepotential of the DC bus 7B is applied to the load 2 when thesemiconductor switching element 10UB or 10VB becomes conductive. Thepower converter main circuit 10 thus outputs two kinds of potentials,i.e., the potential of the DC bus 7A or the potential of the DC bus 7B,thereby operating as a two-level, single-phase inverter circuit.

Each of the semiconductor switching elements 10UA, 10UB, 10VA, and 10VBis formed of a transistor element and a diode element connected inparallel. Note that connection of the diode element in each of theswitching elements may be omitted depending on the characteristic of theload such as when the load is a resistive load.

Although FIG. 13 illustrates a MOSFET as the transistor element, thetransistor element is not limited to the MOSFET but may be any devicethat can switch a state between a low resistance state and a highresistance state by an electrical signal. For example, an IGBT or abipolar transistor may be used as the transistor element. Moreover, awide band-gap semiconductor such as SiC, GaN, or diamond in addition toSi which is used widely may be used as a material of the transistorelement and the diode element making up each of the semiconductorswitching elements 10UA, 10UB, 10VA, and 10VB.

The gate drive circuit 12UA is a first U-phase gate drive circuit thatdrives the semiconductor switching element 10UA that is the firstU-phase power semiconductor switching element, and the gate drivecircuit 12UB is a second U-phase gate drive circuit that drives thesemiconductor switching element 10UB that is the second U-phase powersemiconductor switching element. The gate drive circuit 12VA is a firstV-phase gate drive circuit that drives the semiconductor switchingelement 10VA that is the first V-phase power semiconductor switchingelement, and the gate drive circuit 12VB is a second V-phase gate drivecircuit that drives the semiconductor switching element 10VB that is thesecond V-phase power semiconductor switching element.

Here, among the gate drive circuits 12UA, 12UB, 12VA, and 12VBillustrated in FIG. 13, the configuration of the gate drive circuit 12UAis identical or equivalent to the configuration of the gate drivecircuit 12A according to the first embodiment illustrated in FIG. 1. Onthe other hand, the configuration of each of the gate drive circuits12UB, 12VA, and 12VB omits a detector 12UA4 from the configuration ofthe gate drive circuit 12UA, and is identical or equivalent to theconfiguration of the gate drive circuit 12B illustrated in FIG. 4. Notethat FIG. 13 omits illustration of components corresponding to theinsulating circuits 14A and 14B and the switching signal generation unit20 which are illustrated in FIG. 1.

Note that an operating power supply for operating the detector 12UA4 canalso be used as a power supply for driving the gate drive circuit. Adedicated power supply for operating the detector 12UA4 need not beprovided if the power supply therefor is also used as the power supplyfor driving the gate drive circuit.

In the fourth embodiment, the impedance element group 16 is connectedbetween the gate drive circuit 12UA that is the first U-phase gate drivecircuit and the gate drive circuit 12VA that is the first V-phase gatedrive circuit. That is, FIG. 13 illustrates an example in which theimpedance element group 16 is disposed between the gate drive circuits12UA and 12VA belonging to different phases.

The second main terminal 10UA2 and the second signal input terminal10UA4 of the semiconductor switching element 10UA have the samepotential. Although not illustrated in FIG. 13, the second signal inputterminal 10UA4 of the semiconductor switching element 10UA and one endof the impedance element group 16 are connected to the connection pointof capacitors connected in series (refer to the capacitors 12A5 a and12A5 b in the gate drive circuit 12A in FIG. 1), and thus have the samepotential. These also apply to the semiconductor switching element 10VA.Accordingly, the voltage generated in the impedance element group 16 orthe current flowing therethrough represents a potential differencebetween the second main terminal 10UA2 of the semiconductor switchingelement 10UA and the second main terminal 10VA2 of the semiconductorswitching element 10VA. The gate drive circuits 12UA and 12VA can thusknow the potential difference between the second main terminal 10UA2 ofthe semiconductor switching element 10UA and the second main terminal10VA2 of the semiconductor switching element 10VA by detecting thevoltage or the current from the impedance element group 16.

Note that as one can see from the configuration in FIG. 13, theimpedance element group 16 can be disposed near the gate drive circuits12UA and 12VA. This can shorten a path, namely electrical wiring,through which voltage information or current information from theimpedance element group 16 is transmitted to the gate drive circuits12UA and 12VA, thereby preventing noise from being superimposed on theelectrical wiring. As a result, an effect is obtained where the gatedrive circuits 12UA and 12VA can change the method of driving thesemiconductor switching element 10UA or 10VA with high accuracy by usingaccurate voltage information or accurate current information from theimpedance element group 16.

Next, the operation of the power converter according to the fourthembodiment will be described with reference to FIGS. 13 to 17 asappropriate. Note that the operation of the gate drive circuit will bedescribed with reference to the gate drive circuit 12UA including aswitching speed changing unit 12UA3 and the detector 12UA4. Thedescription assumes that the voltage stored in the capacitor 6 is 1000[V], and the negative terminal of the capacitor 6 is set as thereference potential. That is, the potential at the negative terminal ofthe capacitor 6 is 0 [V].

FIG. 14 is a circuit diagram illustrating one mode of the operation ofthe power converter according to the fourth embodiment. FIG. 14illustrates the mode in which the semiconductor switching elements 10UAand 10VA are on, the semiconductor switching elements 10UB and 10VB areoff, the potential of the output terminal 5U to which the one end of theload 2 is connected matches the potential of the upper DC terminal 8A,and the potential of the output terminal 5V to which the other end ofthe load 2 is connected also matches the potential of the upper DCterminal 8A. Thus, the potential at the second main terminal 10UA2 ofthe semiconductor switching element 10UA is +1000 [V], and the potentialat the second main terminal 10VA2 of the semiconductor switching element10VA is also +1000 [V]. The potential at the second main terminal 10UB2of the semiconductor switching element 10UB is 0 [V], and the potentialat the second main terminal 10VB2 of the semiconductor switching element10VB is also 0 [V].

FIG. 15 is a circuit diagram illustrating one mode of the operation ofthe power converter according to the fourth embodiment, the mode beingdifferent from that of FIG. 14. FIG. 15 illustrates the mode in whichthe semiconductor switching elements 10UA and 10VA are off, thesemiconductor switching elements 10UB and 10VB are on, the potential ofthe output terminal 5U matches the potential of the lower DC terminal8B, and the potential of the output terminal 5V also matches thepotential of the lower DC terminal 8B. Thus, the potential at the secondmain terminal 10UA2 of the semiconductor switching element 10UA is 0[V], and the potential at the second main terminal 10VA2 of thesemiconductor switching element 10VA is also 0 [V]. The potential at thesecond main terminal 10UB2 of the semiconductor switching element 10UBis 0 [V], and the potential at the second main terminal 10VB2 of thesemiconductor switching element 10VB is also 0 [V].

FIG. 16 is a circuit diagram illustrating one mode of the operation ofthe power converter according to the fourth embodiment, the mode beingdifferent from that of each of FIGS. 14 and 15. FIG. 16 illustrates themode in which the semiconductor switching elements 10UA and 10VB are on,the semiconductor switching elements 10UB and 10VA are off, thepotential of the output terminal 5U matches the potential of the upperDC terminal 8A, and the potential of the output terminal 5V matches thepotential of the lower DC terminal 8B. Thus, the potential at the secondmain terminal 10UA2 of the semiconductor switching element 10UA is +1000[V], and the potential at the second main terminal 10VA2 of thesemiconductor switching element 10VA is 0 [V]. The potential at thesecond main terminal 10UB2 of the semiconductor switching element 10UBis 0 [V], and the potential at the second main terminal 10VB2 of thesemiconductor switching element 10VB is also 0 [V].

FIG. 17 is a circuit diagram illustrating one mode of the operation ofthe power converter according to the fourth embodiment, the mode beingdifferent from that of each of FIGS. 14 to 16. FIG. 17 illustrates themode in which the semiconductor switching elements 10UA and 10VB areoff, the semiconductor switching elements 10UB and 10VA are on, thepotential of the output terminal 5U matches the potential of the lowerDC terminal 8B, and the potential of the output terminal 5V matches thepotential of the upper DC terminal 8A. Thus, the potential at the secondmain terminal 10UA2 of the semiconductor switching element 10UA is 0[V], and the potential at the second main terminal 10VA2 of thesemiconductor switching element 10VA is +1000 [V]. The potential at thesecond main terminal 10UB2 of the semiconductor switching element 10UBis 0 [V], and the potential at the second main terminal 10VB2 of thesemiconductor switching element 10VB is also 0 [V].

In the state illustrated in each of FIGS. 16 and 17, a voltage of 1000[V] identical to the capacitor voltage is applied to the impedanceelement group 16. The detector 12UA4 included in the gate drive circuit12UA determines the magnitude relationship between the voltage acrossthe impedance element group 16 representing the capacitor voltageinformation and the reference voltage, and transmits the result of thedetermination to the switching speed changing unit 12UA3. The switchingspeed changing unit 12UA3 can determine whether the capacitor voltage islower or higher than the reference voltage on the basis of the result ofthe determination.

When determining that the capacitor voltage is lower than the referencevoltage, the switching speed changing unit 12UA3 switches control to onethat increases the driving speed of the semiconductor switching element10UA. This control prevents a switching loss of the semiconductorswitching element 10UA. Note that although the above control increasesthe surge voltage generated at the time of switching, the capacitorvoltage is low, so that no excessive voltage is applied to thecomponents inside the power converter main circuit 10. The transition isthus made between the state of FIG. 16 and the state of FIG. 15, andbetween the state of FIG. 17 and the state of FIG. 14.

On the other hand, when determining that the capacitor voltage is higherthan the reference voltage, the switching speed changing unit 12UA3switches control to one that reduces the driving speed of thesemiconductor switching element 10UA. Although the capacitor voltage ishigh, this control reduces the surge voltage generated at the time ofswitching to be able to prevent application of an excessive voltage tothe components inside the power converter main circuit 10.

Note that a storage may be provided in the gate drive circuit as withthe second embodiment illustrated in FIG. 5. FIG. 18 is a circuitdiagram illustrating the configuration of a main part of the powerconverter according to the fourth embodiment, the configuration beingdifferent from that of FIG. 13 and including a storage 12UA5 in the gatedrive circuit 12UA.

In the state illustrated in each of FIGS. 16 and 17, a voltage of 1000[V] identical to the capacitor voltage is applied to the impedanceelement group 16. The detector 12UA4 determines the magnituderelationship between the voltage from the impedance element group 16 andthe reference voltage. When the detector 12UA4 determines that thevoltage from the impedance element group 16 is lower than the referencevoltage, the result of the determination is stored in the storage 12UA5.

The switching speed changing unit 12UA3 switches control to one thatincreases the driving speed of the semiconductor switching element 10UAon the basis of the information determined to be lower than thereference voltage and stored in the storage 12UA5. This control preventsa switching loss of the semiconductor switching element 10UA. Note thatalthough the above control increases the surge voltage generated at thetime of switching, the capacitor voltage is low, so that no excessivevoltage is applied to the components inside the power converter maincircuit 10. The transition is thus made between the state of FIG. 16 andthe state of FIG. 15, and between the state of FIG. 17 and the state ofFIG. 14.

On the other hand, when the detector 12UA4 determines that the voltagefrom the impedance element group 16 is higher than the referencevoltage, the result of the determination is stored in the storage 12UA5.Information stored in the storage 12UA5 is overwritten with a new resultof the determination. That is, the latest result of the determination isstored in the storage 12UA5.

The switching speed changing unit 12UA3 switches control to one thatreduces the driving speed of the semiconductor switching element 10UA onthe basis of the information determined to be higher than the referencevoltage and stored in the storage 12UA5. Although the capacitor voltageis high, this control reduces the surge voltage generated at the time ofswitching to be able to prevent application of an excessive voltage tothe components inside the power converter main circuit 10.

Note that although FIG. 13 illustrates the configuration in which thedetector 12UA4 is provided only in the gate drive circuit 12UA that isthe first U-phase gate drive circuit, the detector may also be providedin the gate drive circuit 12VA that is the first V-phase gate drivecircuit. This configuration allows the gate drive circuit 12VA to changethe method of driving the semiconductor switching element 10VA that isthe first V-phase power semiconductor switching element on the basis ofoutput of the detector. Note that in this configuration as well, astorage may be provided in the gate drive circuit 12VA as in FIG. 18,whereby the effect similar to that of the second and third embodimentscan be obtained.

FIG. 19 is a circuit diagram illustrating the configuration of a mainpart of the power converter according to the fourth embodiment, theconfiguration being different from that of each of FIGS. 13 and 18. Thecharacteristic of the configuration illustrated in FIG. 19 is that twoimpedance element groups 16U and 16V are included as impedance elementgroups, the first impedance element group 16U is connected between thegate drive circuit 12UA that is the first U-phase gate drive circuit andthe gate drive circuit 12UB that is the second U-phase gate drivecircuit, the second impedance element group 16V is connected between thegate drive circuit 12VA that is the first V-phase gate drive circuit andthe gate drive circuit 12UB that is the second U-phase gate drivecircuit, and the gate drive circuit 12UB is provided with a seconddetector 12UB4 b that detects a voltage or current across the firstimpedance element group 16U and a first detector 12UB4 a that detects avoltage or current across the second impedance element group 16V.

The first detector 12UB4 a provided in the gate drive circuit 12UB cantransmit, to a switching speed changing unit 12UB3, capacitor voltageinformation that is obtained from voltage information or currentinformation output from the first impedance element group 16U. Thesecond detector 12UB4 b provided in the gate drive circuit 12UB cantransmit, to the switching speed changing unit 12UB3, capacitor voltageinformation that is obtained from voltage information or currentinformation output from the second impedance element group 16V.

On the basis of the capacitor voltage information, the switching speedchanging unit 12UB3 can perform control to switch or change the drivingspeed of the semiconductor switching element 10UB. Note that the controlfor switching the driving speed is as described above, and thus will notbe described in detail.

According to the configuration of the power converter illustrated inFIG. 19, the opportunity to obtain the capacitor voltage informationincreases in the switching cycle in which the semiconductor switchingelements 10UA, 10UB, 10VA, and 10VB making up the power converter maincircuit 10 are turned on or off, whereby an effect is obtained where themethod of driving the semiconductor switching element 10UB can bechanged with higher accuracy than the configurations described in thefirst to third embodiments.

The configurations illustrated in the aforementioned embodiments merelyillustrate examples of the content of the preset invention, and can thusbe combined with another known technique or partially omitted and/ormodified without departing from the scope of the present invention.

REFERENCE SIGNS LIST

2 load; 5U, 5V output terminal; 6, 6A, 6B capacitor; 7A, 7B DC bus; 8Aupper DC terminal; 8B lower DC terminal; 8C intermediate DC terminal; 10power converter main circuit; 10A1, 10B1, 10C1, 10D1, 10UA1, 10UB1,10VA1, 10VB1 first main terminal; 10A2, 10B2, 10C2, 10D2, 10UA2, 10UB2,10VA2, 10VB2 second main terminal; 10A3, 10B3, 10C3, 10D3, 10UA3, 10UB3,10VA3, 10VB3 first signal input terminal; 10A4, 10B4, 10C4, 10D4, 10UA4,10UB4, 10VA4, 10VB4 second signal input terminal; 10A, 10B, 10C, 10D,10UA, 10UB, 10VA, 10VB semiconductor switching element; 11A, 11Bclamping diode; 12A1 a first on transistor; 12A1 b first off transistor;12A1 c second on transistor; 12A1 d second off transistor; 12A, 12B,12C, 12D, 12UA, 12UB, 12VA, 12VB gate drive circuit; 12A2 a, 12A2 b,12A2 c, 12A2 d gate resistor; 12A5 a, 12A5 b, 12B5 a, 12B5 b capacitor;12A4 a comparator; 12A3, 12B3, 12C3, 12D3, 12UA3, 12UB3 switching speedchanging unit; 12A5, 12B5, 12UA5 storage; 12A4, 12B4, 12C4, 12D4, 12UA4,12UB4 a, 12UB4 b detector; 12A4 b, 12A4 c resistance element; 14A, 14Binsulating circuit; 14A1 light-emitting diode; 14A2 phototransistor; 16,16A, 16B, 16U, 16V, 18 impedance element group; 16 e 1 to 16 e 10, 18 e1 to 18 e 10 impedance element; 20 switching signal generation unit.

1. A power converter comprising: a power converter main circuitincluding two or more semiconductor switching elements; gate drivecircuits, each of which drives a corresponding one of the semiconductorswitching elements; and one or a plurality of impedance elementsconnected between at least one pair of the gate drive circuits, whereinat least one of the gate drive circuits includes a detector to detect avoltage across the impedance elements or a current flowing through theimpedance elements, and changes a driving speed of the semiconductorswitching elements in accordance with an output of the detector.
 2. Thepower converter according to claim 1, wherein the power converter maincircuit includes a multi-level circuit corresponding to one phase or aplurality of phases to select a potential of any one of two or more DCterminals and output the selected potential to a load, the multi-levelcircuit includes two or more semiconductor switching elements connectedin series, and the impedance element is disposed between the gate drivecircuits belonging to a same phase.
 3. The power converter according toclaim 2, wherein the multi-level circuit is a two-level circuit toselect a potential of either one of an upper DC terminal and a lower DCterminal and output the selected potential to the load, the two-levelcircuit includes a first semiconductor switching element and a secondsemiconductor switching element sequentially connected in series betweenthe upper DC terminal and the lower DC terminal, and a connection pointof the first semiconductor switching element and the secondsemiconductor switching element is connected to the load.
 4. The powerconverter according to claim 2, wherein the multi-level circuit is athree-level circuit to select a potential of any one of an upper DCterminal, an intermediate DC terminal, and a lower DC terminal andoutput the selected potential to the load, the three-level circuitincludes: first, second, third, and fourth semiconductor switchingelements sequentially connected in series between the upper DC terminaland the lower DC terminal; a first diode element connected between theintermediate DC terminal and a connection point of the firstsemiconductor switching element and the second semiconductor switchingelement; and a second diode element connected between the intermediateDC terminal and a connection point of the third semiconductor switchingelement and the fourth semiconductor switching element, and a connectionpoint of the second semiconductor switching element and the third powerswitching semiconductor element third semiconductor switching element isconnected to the load.
 5. The power converter according to claim 1,wherein the power converter main circuit includes a multi-level circuitcorresponding to a plurality of phases to select a potential of any oneof two or more DC terminals and output the selected potential to a load,the multi-level circuit includes two or more semiconductor switchingelements connected in series for each phase, and the impedance elementis disposed between the gate drive circuits belonging to differentphases.
 6. The power converter according to claim 5, wherein themulti-level circuit is a two-level circuit to select a potential ofeither one of an upper DC terminal and a lower DC terminal and outputthe selected potential to the load, the two-level circuit includes afirst semiconductor switching element and a second semiconductorswitching element sequentially connected in series between the upper DCterminal and the lower DC terminal, and a connection point of the firstsemiconductor switching element and the second semiconductor switchingelement is connected to the load.
 7. The power converter according toclaim 5, wherein the multi-level circuit is a three-level circuit toselect a potential of any one of an upper DC terminal, an intermediateDC terminal, and a lower DC terminal and output the selected potentialto the load, the three-level circuit includes: first, second, third, andfourth semiconductor switching elements sequentially connected in seriesbetween the upper DC terminal and the lower DC terminal; a first diodeelement connected between the intermediate DC terminal and a connectionpoint of the first semiconductor switching element and the secondsemiconductor switching element; and a second diode element connectedbetween the intermediate DC terminal and a connection point of the thirdsemiconductor switching element and the fourth semiconductor switchingelement, and a connection point of the second semiconductor switchingelement and the third semiconductor switching element is connected tothe load.
 8. The power converter according to claim 1, wherein the gatedrive circuit increases the driving speed of the semiconductor switchingelement when a voltage transmitted from the impedance element to thegate drive circuit is lower than a reference voltage.
 9. The powerconverter according to claim 1, wherein the gate drive circuit reducesthe driving speed of the semiconductor switching element when a voltagetransmitted from the impedance element to the gate drive circuit ishigher than a reference voltage.
 10. The power converter according toclaim 1, wherein at least one of the gate drive circuits includes astorage to store information on whether a voltage transmitted from theimpedance element is higher or lower than a reference voltage, and thegate drive circuit that includes the storage changes the driving speedof the semiconductor switching element on a basis of the informationstored in the storage.